1. Field of the Invention
The invention relates to multi-channel nerve integrity monitoring, and more particularly to the patient connection electrodes, their transmission lines and a manner by which these components may communicate to the main monitoring unit to actuate automatic setup functions.
2. Description of Related Art
Despite advances in diagnostic, microsurgical and neurological techniques that enable a more positive anatomical identification of facial nerves, following surgical procedures to the head and neck, such as an acoustic neuroma resection, there is a significant risk of a patient losing facial nerve function. Because of the very delicate nature of these facial nerves, even the best and most experienced surgeons using the most sophisticated equipment known, encounter a considerable hazard that one or several nerves will be bruised, stretched or severed, during an operation.
However, studies have shown that preservation of facial nerves during an acoustic neuroma resection, for example, may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Broadly stated, the locating procedure, also known as nerve integrity monitoring, involves inserting sensing or recording electrodes directly within the cranial muscles innervated or controlled by the nerve of interest. Such an exemplary monitoring electrode is disclosed in U.S. Pat. No. 5,161,533 to Prass et al., which is incorporated herein by reference in its entirety.
One method of nerve localization involves the application of electrical stimulation near the area where the subject nerve is believed to be located. If the stimulation probe contacts or is located in the area reasonably close to the nerve, the stimulation signal applied to the nerve is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle which is then transferred to the recording electrodes, thereby providing an indication to the surgeon as to the location of the nerve. Stimulation is accomplished using handheld monopolar or bipolar probes, such as the Electrical Stimulus Probe described in U.S. Pat. No. 4,892,105 to Prass, which is incorporated herein by reference in its entirety.
The Electrical Stimulus Probe (now known as the "Prass Flush-Tip Monopolar Probe") is insulated up to the distal tip to minimize current shunting through undesired paths. Another example of a bipolar probe is described in the U.S. Provisional Patent Application Ser. No. 60/096,243, entitled "Bipolar Electrical Stimulus Probe", filed Aug. 12, 1998, which is incorporated herein by reference in its entirety.
Another method of nerve localization involves the mechanical stimulation of the nerve of interest by various dissecting instruments. Direct physical manipulation of a motor nerve may cause the nerve to conduct a nerve impulse to its associated musculature. If those muscles are being monitored using a nerve integrity monitoring instrument, the surgeon will hear an acoustic representation of the muscle response in close temporal relationship to the antecedent mechanical stimulation. This will allow the nerve of interest to be roughly localized at the contact surface of the dissecting instrument.
Prior art nerve integrity monitoring instruments (such as the Xomed.RTM. NIM-2.RTM. XL Nerve Integrity Monitor) have proven to be effective for performing the basic functions associated with nerve integrity monitoring, such as recording Electromyogram (EMG) activity from muscles innervated by an affected nerve and alerting a surgeon when the affected nerve is activated by application of a stimulus signal. However, these nerve integrity monitoring instruments have significant limitations for some surgical applications and in some operating room environments, as discussed below.
For example, a significant limitation in the majority of prior art nerve integrity monitoring devices is the availability of only two channels for monitoring of EMG activity. This two channels monitoring capability provides a limited ability to monitor multiple nerves or multiple branches of single nerves. In addition, a limited number of channels does not allow for redundancy in the event of electrode failure.
In some cases, such as with monitoring the facial nerve during the performance of parotidectomy, monitoring must be performed from each of four major branches of the facial nerve. Alternatively, procedures involving the more proximal (closer to the brainstem origin) portion of the facial nerve may be effectively monitored by a single channel, in that the nerve exhibits no topographical organization in that location. With only two channels available, there is also limited ability to distinguish whether recorded signal events represent artifacts or EMG responses, based upon their distribution among "intelligent" and "non-intelligent" electrodes, as described in U.S. patent application Ser. No. 09/213,015, filed on Dec. 16, 1998. That is, true or important EMG signals provoked by surgical manipulations distribute "intelligently" only to muscles supplied by the nerve of interest. In contrast, electrical artifacts distribute "nonintelligently" to all proximate electrodes within an electrical or electromagnetic field. Thus a multi-channel recording capability allows the user to distinguish artifacts and EMG signal events on the basis of such distribution.
Another advantage of multi-channel recording is that, with the availability of some redundancy, different recording strategies may be used for recording signals from the muscles supplied by a single nerve of interest, in order to take advantage of their respective advantages and minimize their inherent disadvantages. The most commonly used recording method for intraoperative nerve integrity monitoring involves intramuscular placement of two closely spaced electrodes. The use of intramuscular electrodes in close bipolar arrangement (as described in U.S. Pat. No. 5,161,533) is preferred in order to obtain adequate spatial selectivity and maintenance of high common mode rejection characteristics in the signal conditioning pathway for reduced interference by electromagnetic artifacts. Such electrode configurations yield a compressed dynamic range of electrical voltage observed between the paired electrodes. For example, if it is physically situated near one of the electrodes, a single motor unit (activation of a single nerve fiber) may cause an adequate voltage deflection to be heard as a clear signal spike or to exceed a predetermined voltage threshold. Moreover, with close electrode spacing and bipolar amplification, recording of larger responses is frequently associated with internal signal cancellation, which significantly reduces the amplitude of the observed electrical signal. The resultant compressed dynamic range is advantageous for supplying direct or raw EMG signal feedback to the operating surgeon, in that both large and small signal events may be clearly and comfortably heard at one volume setting. However, the method offers a limited ability to fractionate responses based upon their overall magnitude.
For quantitative measurements of EMG response amplitudes, a preferential recording method involves the use of surface electrodes in a monopolar arrangement, with an active electrode placed over a suitable muscle, supplied by the nerve of interest, and the other electrode placed at a relative distance away in an electrically neutral site. The active electrode summates muscle activity over a greater or more representative area than intramuscular electrodes and the absence of a simultaneous signal in the inactive ("indifferent reference") electrode eliminates unpredictable signal cancellation seen in bipolar recording where both electrodes in a pair detect the same signal from different perspectives. Measurement of the response amplitude using this recording method provides an excellent representative measure of relative magnitude of muscle activation. However, the monopolar ("indifferent reference") arrangement with surface electrodes provides a poor quality signal for acoustic (loudspeaker) feedback to the operating surgeon. With multi-channel recording capability, this method of EMG recording may be employed in parallel with closely spaced intramuscular electrodes in order to achieve better signal quality.
With the stated potential advantages afforded by multi-channel recording capability, some devices are known to include up to eight channels of EMG recording capability. However, while multi-channel recording affords the possible advantages stated above, it poses a significant disadvantage of requiring greater complexity of off-line diagnostic or system check, and recording setup procedures. This is especially true if certain channels are designated for quantitative purposes using a monopolar method and others are used for feedback to the operating surgeon with closely spaced intramuscular electrodes. Thus, a method is needed that would allow a surgeon to take advantage of the flexibility afforded by a multi-channel EMG recording capability, while reducing the relative complexty of setup and diagnostic functions.
Another problem posed by the availability of a greater number of EMG recording channels, in the setting of coventional art, is that all electrodes are provided individually or in kits with separate connectors for each lead. Furthermore, "protected" pin connectors required by conventional devices are more bulky and cumbersome to use than the "standard" pin connectors. Regardless of color coding and other labeling strategies, with multiple recording electrodes, placing the connector pins in the electrode receiving portion ("head box") of the nerve integrity monitor can be quite tedious, time consuming, and confusing, such this configuration may result inaccuracy regarding proper placement of the connectors.
Another problem related to multiple channel recording is that the head box portion of the nerve integrity monitoring, which receives the electrodes placed in various locations on the patient, must be of a sufficient size to accommodate all of the necessary connections. The larger size of the head box may render it more susceptible to electromagnetic noise and may be too large to allow it to be placed physically near the area where the electrodes are placed. Multiple channel head boxes are typically placed under the operating table, because they cannot be placed close to the electrode sites.
Accordingly, the remote location of the head box results in electrode leads being typically one meter in length or longer. Electrode leads are typically unshielded from the effects of electromagnetic noise, and the longer the length of the leads, the more susceptible they are to such interference. One method of improving the resistance of electrode leads to electromagnetic noise is to arrange them in a "twisted-pair" fashion, as described in U.S. Pat. No. 5,161,533. Such an arrangement better preserves common mode rejection capabilities within the signal path than otherwise untreated leads. Therefore, a method that allows reduction of the size of the head box apparatus or otherwise further reduces the potential electromagnetic interference in the electrode leads would be desired.